Sweetening process



; various degrees with basic materials.

Patented Oct. 23, 1951 SWEETENING PROCESS Harold A. Ricards, Jr.,Cranford, and James W. Ryder, Watchung, N. J assignors to Standard OilDevelopment Company, a corporation of Delaware Application February 16,1949, Serial No. 76,802

3 Claims.

The present invention is concerned with an improved sweetening process.The invention is more particularly concerned with the removal ofmercaptan compounds, particularly those mercaptan compounds which haveheretofore been relatively difficult to remove from hydrocarbonfractions especially from those fractions boiling in the motor fuel,kerosene, diesel oil and heating oil boiling ranges. In accordance withthe present invention a feed hydrocarbon oil characterized by containingmercaptan compounds, various phenolic compounds and other acidiccompounds, such as lower molecular weight aliphatic acids, is contactedin a series of treating zones wherein a particular arrangement andsequence of the stages are employed. In accordance with the broadestconcept of the present process a feed oil containing mercaptan compoundsis contacted in a primary zone with a relatively concentrated causticsolution under conditions to remove from the feed oil objectionablecompounds as for example phenolic compounds. The oil is then contactedin a secondary zone with a less concentrated caustic solution than thatemployed in the primary zone and with solubility enhancing agents underconditions to remove from the oil the objectionable mercaptan compounds.One embodiment of the present invention is to process a feed oil whichis characterized by containing hydrogen sulphide, phenolic typecompounds, mercaptans and organic carboxylic acids, to remove from theoil the hydrogen sulphide. The hydrogen-sulphide free oil is thencontacted in an initial stage with a relatively weak caustic solution.The oil is further treated in a secondary stage with a relatively strongcaustic solution and is finally contacted in a tertiary stage with acaustic solution of an intermediate concentration. Solubility enhancingagents recovered from the spent caustic phase segregated from theinitial stage are used in conjunction with the intermediate causticsolution utilized in the tertiary stage.

It is well known in the art to treat petroleum oils by variousprocedures in order to remove objectionable compounds, as for examplesulfur compounds therefrom. For example, it is known to treat petroleumfractions boiling in the motor fuel boiling range and in the generalrange below about 700 F. with various chemical reagents such as sodiumor potassium hydroxide solutions.

Also it has long been known that weakly acidic materials, such asmercaptans, are reactive to One method for the removal of mercaptansIIOIIL hydrocarbon streams has been to treat the streams with a reagentwhich is insoluble in the hydrocarbon stream but which contains alkalitype of material which reacts with mercaptans. Such treating reagentshave normally been aqueous solutions. The reaction product is a basicsalt of the mercaptan. Normally such salts exhibit some solubility inthe aqueous phase, this'solubility decreasing as the molecular weight orbranchiness of the hydrocarbon structure of the mercaptan increases. Theart has generally recognized in the treating of hydrocarbon streams withaqueous solutions such as strong or weak I caustic in a continuous typeprocess, that the equilibrium is generally unfavorable for mercaptanextraction. Therefore, caustic solutions when they are only spent to asmall extent in mercaptide salts will not give an 'further reduction inmercaptan content despite the fact that these treating solutions have alarge'residual free caustic content. The art has also recognized thatthe efficiency of the spent caustic can be part ally or fully restoredby regeneration of these solutions. The regeneration normally consistsof removal of a part or all of the mercaptide salts by means ofhydrolysis and vaporization or by oxidation.

One method is to employ steam regeneration for the revivification ofcaustic solution spent in mercaptan removal from hydrocarbon streams.The steam regeneration is normally carried out in a packed tower or atower provided with bubble plates or pierced type tray plate design. Thesteam hydrolizes the mercaptide "salt to the corresponding mercaptanswhich are stripped out by the steam vapors and taken overhead. Theregenerator also serves as a means of adjusting the gravity of thecaustic solution, which has been shown to be critical. The regeneratedcaustic is withdrawn from the bottom of the regeneration tower andreused in the extraction stage of the system wherein the causticcontacts the hydrocarbonstream through suitable mixing devices forfurther extraction of mercaptans' by the same caustic.

It has also been found that certain constituents of the oil itself willfunction as solubility enhancing agents. These constituents tend toimprove the partition coefficient of the acid mercaptan in the aqueousphase and to increase the solubility of the mercaptide salts in theaqueous phase, particularly when the mercaptan is of a relatively highmolecular weight. As mentioned, mercaptans are diflicult to extract fromhydrocarbon solutions by straight aqueous alkali solutions. This isparticularly true of the higher molecular weight mercaptans. In general,this is occasioned by the fact that the two phases are not miscible andthat good contact must be obtained to allow the alkali metal ion toreact with the mercaptan hydrocarbon. Furthermore, once the reaction hasoccurred, it is essential that if the extraction of the mercaptan is tobe accomplished the reaction product must be soluble in the aqueouslayer. As pointed out heretofore, the higher the molecular weight of themercaptan the lower the solubility of the corresponding mercaptide saltin the aqueous phase. The solubility enhancing agents tend to increasethis solubility and thus increase the effectiveness of the alkalitreating solution.

The exact chemical nature of these solubility enhancing agents isnotentirely known. However, it is felt that they comprise the lowermolecular weight aliphatic acids and the various *substituted phenoliccompounds. It has been found that phenolic type materials having alkylside chains containing less than about 3 carbon atoms are not effectiveby themselves as solubility promoters and that organic carboxylic acidshaving less than about 3 carbon atoms per molecule are like-wise notvery effective. However, as the length of the side chain'of the phenolsincreases or the length of the aliphatic acid hydrocarbon chainincreases, their effectiveness as solubility promoters increasesgreatly. However, the solubility of the acids or phenols themselves inthe caustic solution decreases with increasing hydrocarbon chain lengthand thus limits the molecular weight of acid or phenolic -type materialwhich can be employed. It is be- :lieved that the optimum chain lengthshould be from about 3 carbon to '7 carbon atoms for the acids." V

' However, with respect to naturally occurring solubility enhancingagents, it has been found that the effectiveness of the respectivesolubility en- -hancing'agents when used in conjunction with alkalisolutions varies appreciably Asa matter "of fact, it has been discoveredthat in many in- .stances certain so-called solubility enhancing agentsdo not produce any better result than that'obtained with a pure causticsolution alone. It has, however, now been discovered that providing afeed oil stream is treated in a particular manner so as to segregate.desirable solubility enhancing agents to the exclusion of agents whichfunction to counteract the effect ofthe solubility enhancing agents,unexpected and desirable results are secured. As pointed out heretofore,the broad concept of the present invention is to treat afeed oil in aninitial stage of a twostage process with a relatively strong causticsolution and then to treat the oil in a secondary stage witha lessconcentrated caustic solution in the presence of naturally occurringsolubility enhancing agents. In a specific concept of the presentinvention a feed oil is treated in an initial stage with a relativelydilute caustic solution. The oil is subsequently treated in a secondarystage with arelatively concentrated caus- --tic solution and is finallytreated in a tertiary stage with an intermediate strength causticsolution' containing certain solubility promoting agents to enhancemercaptan extraction.

Theprocess of the present. invention may be readilyunderstood byreferring to the drawing illustrating an embodiment of the same.Referring specifically to the drawing a feed. oil which .for the'purposeof illustration is assumed to. be

a petroleum fraction boiling in the range from about to 420 F. isintroduced into a hydrogen sulphide treating zone I by means of feedline 2. In zone I the feed oil is processed in a manner adapted toremove substantially completely the hydrogen sulphide from the feed oil.The treating reagent, if one be employed, is introduced into zone I bymeans of line 3 and withdrawn by means of line 4. It is to be understoodthat if the feed oil is free of hydrogen sulphide, this preliminarytreatment in zone I may be dispensed with. A very desirable method ofremoving hydrogen sulphide is by a distillation operation. Thedistillation operation may be carried out by any suitable means.However, when processing a feed oil boiling in the motor fuel boilingrange, it is desirable to remove the hydrogen sulphide in astabilization zone comprising a depropanizer and debutanizer. Generally,the hydrogen sulphide is removed with the propane in a depropanizingtower prior to stabilizing the motor fuel in a debutanization zone.

The feed oil substantially completely free of hydrogen sulphide iswithdrawn from zone I by means of line 5 and introduced into an initialcaustic treating zone ID. In zone II! the oil is contacted with arelatively dilute caustic solution which for the purpose of illustrationis assumed to be a dilute sodium hydroxide solution. The dilute sodiumhydroxide solution is introduced into zone In by means of line 6 and iswithdrawn by means of line I and handled as hereinafter described. g a

The treated oil is removed from stage In by means of line 8 and isintroduced into a secondary stage 20 wherein the same is contacted witharelatively concentrated sodium hydroxide solution. The sodium hydroxidesolution is introduced into stage or zone 20 by means of line 9 and iswithdrawn by means of line I I.

The treated oil is removed from stage 2!] by means of line I2 and isintroduced into a tertiary stage 30, wherein the same is contacted witha sodium hydroxide solution having an intermediate concentration andcontaining solubility enstage I0 by means of line I is preferably passedinto a reconcentration zone 40 and handled in a manner to remove watertherefrom. The concentrated caustic is removed from zone 40 by means ofline I l while the water is removed overhead by means of line I 3. Theconcentrated caustic solution containing valuable solubility enhancingagents or sodium salts is introduced into the caustic solution passinginto tertiary stage 30 by means of line 32 and line 26. Under certainconditions it may be desirable to add fresh sodium hydroxide to theconcentrate removed from zone 40. The sodium hydroxide can be added bymeans of line 2 I.

Although it is preferred to operate stage I0 continuously, under certainconditions satisfactory operation might be secured if this zone beoperated intermittently. It isv also within the scope uents.

weight aliphatic acids.

the segregation of the solubility enhancing agents of the presentinvention is a feed oil from a naphthenic crude which has been thermallycracked. The cracking temperatures are above about 750 F. and preferablyin the range from about 850 F. ,to 950 F. 200 lbs. per sq. in. andpreferably in the range from about 750 to 1000 lbs. per sq. in.

of the present invention to recover these desirable solubility enhancingagents by the segregation of the spent caustic stream withdrawn fromstage In and by the acidification of the same. The acid oil phasecontaining the segregated solubility enhancing agents may then be addeddirectly to partially recycled b means of line 3|. Th spent causticsolution removed from tertiary stage 30 by means of line 22 isintroduced into zone 50 wherein the mercaptides are reconverted tomercaptans by hydrolysis and are removed overhead by means of line 24.Steam is introduced into zone 60 by means of line 23. The regenerationoperation conducted in zone 60 also is controlled to adjust the gravityof the regenerated solution. The regenerated caustic may be removed fromthe system byimeans of line 25 but is preferably recycled in a manner ashereinafter described to tertiary zone 30 by means of lines 26 and I5.

The invention is broadly directed toward a two stage process wherein afeed oil is contacted with a relatively concentrated caustic solution inthe initial stage and with a less concentrated caustic solution in asecond stage. The treatment in the initial stage serves to removevarious constituents as .for example various low molecular weightphenolic compounds and related constituent which would otherwise serveto greatl adversely affect the efficiency of the solubility enhancingagents used in conjunction with the caustic treating solution employedin the second stage. A more specific embodiment of the inventioncomprises 'a three stage operation wherein a relativel weak causticsolution is employed in the initial stage; a relatively concentratedsolution of caustic employed in the secondary stage and a causticsolution of intermediate strength utilized in a tertiary stage inconjunction with solubility enhancing captan compounds from hydrocarbonfractions which boil in the motor fuel, kerosene, diesel oil and gas oilboiling ranges. Particularly desirable feed oils are those fractionsderived from naphthenicor aromatic type crudes and which containnaphthenic acids or other similar constit- These naphthenic acids uponcracking apparently form various related low molecular A preferred feedoil for Cracking pressures are above about While the present inventionmay be used in the treatment of any type of feed oil, as for example;for the treatment of a mixed cracked and virgin fraction, it isapparent that the present invention is particularly adapted for theprocessing of .cracked fractions derived from aromatic type J crudescontaining naphthenic acids. It is essential in accordance with thepresent invention that hydrogen sulphide be substantially completelyremoved from the feed oil in a preliminary treating zone. By operatingin the manner described a more efficient use of naturally occurringsolubility enhancing agents is secured.

The feed oil which has been freed of hydrogen sulphide and which ispreferably a cracked fraction derived from an aromatic or naphthenictype crude is contacted in an initial caustic treating zone wherein itis contacted with a relatively dilute caustic solution, as for examplewith a dilute sodium hydroxide solution.

The fresh sodium hydroxide employed in the initial caustic treating zoneshould have a B. gravity below about 10 and preferably have a gravity inthe range from about 1 to 5. Thus, the concentration of the sodiumhydroxide in the fresh caustic solution should be below about 7% and ispreferably in the range from about .7 to 3 /2%. When the concentrationof the caustic solution is specified in the present invention, it ismeant for example, the grams of sodium hydroxide present in a hundredgrams of mixture comprising sodium hydroxide and water. By utilizing asolution of this character it has been found that the most effectivesolubility enhancing agents can be readily removed from the oil.Furthermore, it has been found that when utilizing a caustic solutionhaving these critical concentrations the various phenolic compounds andother constituents which apparently greatly adversely affect theefiiciency of the solubility enhancing agents are not picked up by thecaustic solution but remain in the oil stream being treated.

Thus, a very sharp selective separation is made between the desiredsolubility enhancing agents which are picked up in the caustic solutionand the other acidic constitutents which adversely affect the efficiencyof the solubility enhancing agents. While a once through operation maybe employed with respect to the caustic stream, it

.the various factors so that the spent caustic stream withdrawn from theinitial zone and not recirculated is spent to a degree in the range fromabout 30% to 50%. It is to be understood that the term spent when usedin the present invention means that percentage of sodium ion which hasreacted with organic carboxylic acids,

.as for example with aliphatic acids, to produce sodium salts of theseacids. In the operation, upon initial contact a portion of the sodiummight react with the various phenols to form sodium phenolates. However,upon recirculation the phenolates are reconverted to the phenols and aredisplaced from the aqueous solution with the resulting formation ofsodium salts of aliphatic acids. Thus, the amount of sodium not spentmay be available either in the form of the hydroxide or as a sodiumphenolate. Although the amount of sodium available may be entirelypresent as a sodium phenolate, it is preferred that a small amount ofthe sodium also be available as the hydroxide.

The concentration of the caustic specified in the present invention'withrespect to the initial stage is the concentration at which effectiveresultsare secured. caustic solutions could be initially used, as for Itis obvious that stronger example 10 B. or higher and somewhat similarresults obtained by recirculating the spent caustic until from about 70%to 100% is spent, that is, until 70% to 100% of the sodium ion isreacted to form sodium salts of aliphatic acids. 'When employing causticsolutions having greater concentrations than about 7 'B., it ispreferred that the caustic solution withdrawn from the initial stage bespent in excess of about 90%.

The strength of the fresh caustic added in the secondary caustictreating stage is preferably in the rangefrom about 24% to 30% NaOH.Here again although a once through operation may be utilized, it ispreferred to employ at least partial recycling under conditions so thatthe total caustic stream utilized in the treatment of the oil in thesecondary stage does not have a concentration less than about 12% to 15%NaOH.

'If the s ent caustic stream has approximately preferred that in theinitial and in the secondary stages the amount of caustic solutionemployed based upon the oil be sufficient to obtain good mixing andremove desired acidic constituents,

which by volume is in the range from about to 50%.

'The concentration of the caustic solution employed in the tertiarystage is less than the concentration of the caustic used to contact theoil inthe secondary stage. As previously mentioned the concentration ofthe caustic solution employed in the secondary stage is preferably inthe range from about 12% to 15%. The concentration of the causticemployed in the tertiary stage for any particular operation is less thanthese specified concentrations for the secondary stage, and is in thegeneral range from about It is preferred that the concentration of thecaustic employed in the tertiary stage be from about 1% to 3% less thanthe caustic employed in the secondary stage.

These desired concentrations of the caustic for utilization in thetertiary stage may be secured by controlling the amount of fresh causticadded and the extent to which the spent caustic is regenerated. Anotherfactor which must be taken into consideration in adjusting the desiredstrength of caustic in the tertiary stage is the amount of water addedwith the spent caustic containing the solubility enhancing agents fromthe initial stage. The amount of caustic solution employed in thetertiary stage is in the range from about 5 to 50% based upon the volumeof oil being contacted. The concentration of the solubility enhancingagents present in the caustic solution introduced into the tertiarystage is preferably in the range so that from about 25% to 75% of thetotal sodium or other alkali metal is present as a sodium salt.

By operating in the manner described and f controlling the concentrationof the caustic thereto of fresh sodium hydroxide having a higherconcentration'of sodium hydroxide. A

"stream; phenolic'and other acidic constituents which were "not removedfromthe oil in the secondary stage will not be removed from'the'oil inthe tertiary stage. Thus, these objectionable phenolic compounds andother acidic constituents will not build up or accumulate inthe-circulating caustic stream of the tertiary stage with a resultingimpairment of eiiiciency with respect to the removal of mercaptancompounds from the oil stream. The spent caustic stream may berecirculated to a 'revivification or regeneration unit as hereinbeforedescribed.

The caustic stream withdrawn from the? initial zone may be handled toconcentrate the same in a concentration unit. If this procedure beadopted the extent-to which the stream is concentrated generally willdepend upon the concentration of caustic desired in the stream being fedto the tertiary zone. Another method of handlingthis stream is tofortify the same by the addition still further method of hanc'ling thisspent caustic stream is to introduce it directly into-theregenerationzone with the spent caustic withdrawn from the tertiary zone.Furthermore, as pointed out heretofore, this initial stage maybeoperated intermittently. The desirable solubility enhancing agents maybe segregated from the stream by the acidification of the spent causticstream resulting in the formation of an oily layer containing thedesirable compounds.

The invention may be further understood by the following exampleillustrating one embodiment of the same.

Example A hydrocarbon feed oil secured from the thermal crackingofa'naphthenic type gas-oil and reduced crude (Quiriquire Crude) andboiling in the range from about F.'to 450 F. was processed in accordancewith the present invention. Theinspections of the oil treated were asfollows:

Copper number, mg. mercaptan sulfur/ ml '41 vPhenol number, mg. tertiaryamyl phenol/ 100 m1 375 Gravity, API 57.0 Neutralization number, mg.KOH/g 1.17

Reactive sulfur, mg./ 100 m1 Trace Total sulfur, 0.228

The distribution of mercaptans in the naphtha was as follows:

This naptha was treated in a manner to completely free the oil ofhydrogen sulphide. This was accomplished in a stabilization anddebutanization operation. The oil was then treated in an initial stagewith a caustic solution having a concentration of about 3% NaOH and in asecondary stage with a caustic solution having-a concentrationof about17% NaOH. The oil was contacted in a tertiary stage in the mannerdescribed. with a caustic solution having aconcentration ofabout'14%'NaOH and solubility enhancing agents equivalent to 10% NaOH. The treatedstock had a, copper number (milligrams mercaptan S/100 mililiters) ofabout one. The amount Of caustic employed in the respective stages Wasabout by volume based upon the volume of oil contacted. In thisoperation the caustic solution segregated from the first stage wasconcentrated to about 12% sodium hydroxide and was utilized inconjunction with fresh caustic in the tertiary stage.

In another operation similar to that described above, except thatapproximately the same concentration of caustic was employed in thetertiary stage as in the secondary stage, the phenol number of thecaustic withdrawn from the tertiary stage built up with the result thatthe copper number of the finished treated oil was approximately 6 to 8.

It is to be understood that the present invention may be employed in thetreatment of various petroleum fractions other than motor fuels althoughit is particularly adapted for the production of high quality motorfuels. Heating oils, kerosenes, and the like may be treated in a manneras described. Although the invention has been particularly describedwith respect to the use of a sodium hydroxide solution, potassiumhydroxide, and other alkali metal hydroxide solutions may also be used.It is also to be understood that the solubility enhancing agents of thepresent invention may be segregated in accordance with the presentprocess and used in conjunction with caustic solutions employed in thetreatment of other hydrocarbon streams. Thus, for example, it is withinthe scope of the present invention to segregate solubility enhancingagents by means of a weak caustic solution (below about 7%), from anaphthenic type crude, particularly from a feed oil which has beenthermally cracked and derived from a naphthenic type crude and to usethese solubility enhancing agents with caustic in the treatment of otherstreams. It is within the scope of the present invention to segregatethe solubility enhancing agents of the present invention from a feed oilwhich has been thermally cracked and derived from a naphthenic typecrude for the caustic treatment of a feed oil "which has beencatalytically cracked.

Having described the invention, it is claimed:

1. Process for the removal of mercaptan compounds from hydrocarbonmixtures containing the same which comprises contacting the hydrocarbonfraction in an initial stage with a relatively weak caustic solutionhaving a concentration of about 0.7 to 7%, whereby agents are extractedby the caustic which increase the solubility of mercaptans in caustic,removing the caustic solution from said hydrocarbon fraction andcontacting the hydrocarbon fraction in a secondary stage with arelatively strong caustic solution having a concentration of about 12 to15%, removing the caustic solution therefrom and treating the oil in atertiary stage with an intermediate strength caustic solution having aconcentration about 1 to 3% lower than the concentration employed in thesaid secondary stage and including therewith at least a portion of thecaustic removed from said initial stage containing said agents toincrease solubility of mercaptans in caustic.

2. The process defined by claim 1 in which said caustic withdrawn fromsaid initial stage is concentrated prior to introducing the same intosaid tertiary stage.

3. Improved process for the removal of mercaptan compounds from a feedoil which is characterized by containing hydrogen sulphide and mercaptancompounds which comprises processing said feed oil in a treating zone tosubstantially completely remove the hydrogen sulphide from said feedoil, then contacting said hydrogen sulphide-free feed oil with arelatively weak caustic solution having a concentration of about 0.7 to7%, whereby agents are extracted by the caustic which increase thesolubility of mercaptans in caustic, removing the caustic solution fromsaid feed oil and contacting the feed oil in a secondary stage with arelatively strong caustic solution having a concentration of about 12 to15 removing the caustic solution therefrom and treating the oil in atertiary stage with an intermediate strength caustic solution having aconcentration about 1 to 3% lower than the concentration employed in thesaid secondary stage and including therewith at least a portion of thecaustic removed from said initial stage containing said agents toincrease solubility of mercaptans in caustic.

HAROLD A. RICARDS, JR. JAMES W. RYDER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,273,104 Heilman Feb. 17, 19422,297,866 Benedict Oct. 6, 1942 2,393,476 Messmore Jan. 22, 19462,431,770 Payne et a1 Dec. 2, 1947 2,474,028 Berger June 21, 19492,478,603 Stratton Aug. 9, 1949

1. PROCESS FOR THE REMOVAL OF MERCAPTAN COMPOUNDS FROM HYDROCARBONMIXTURES CONTAINING THE SAME WHICH COMPRISES CONTACTING THE HYDROCARBONFRACTION IN AN INITIAL STAGE WITH A RELATIVELY WEAK CAUSTIC SOLUTIONHAVING A CONCENTRATION OF ABOUT 0.7 TO 7%, WHEREBY AGENTS ARE EXTRACTEDBY THE CAUSTIC WHICH INCREASE THE SOLUBILITY OF MERCAPTANS IN CAUSTIC,REMOVING THE CAUSTIC SOLUTION FROM SAID HYDROCARBON FRACTION ANDCONTACTING THE HYDROCARBON FRACTION IN A SECONDARY STAGE WITH ARELATIVELY STRONG CAUSTIC SOLUTION HAVING A CONCENTRATION OF ABOUT 12 TO15%, REMOVING THE CAUSTIC SOLUTION THEREFROM AND TREATING THE OIL IN ATERTIARY STAGE WITH AN INTERMEDIATE STRENGTH CAUSTIC SOLUTION HAVING ACONCENTRATION ABOUT 1 TO 3% LOWER THAN THE CONCENTRATION EMPLOYED IN THESAID SECONDARY STAGE AND INCLUDING THEREWITH AT LEAST A PORTION OF THECAUSTIC REMOVED FROM SAID INITIAL STAGE CONTAINING SAID AGENTS TOINCREASE SOLUBILITY OF MERCAPTANS IN CAUSTIC.